ANATOMY OF LEAVES. 2S5 



description, the sessile leaves of monocotyledonous 

 plants. On examining it, we find that the vascular 

 frame work consists of a distinct midrib, which forms 

 the keel of the leaf, and of less elevated ribs (costa) 

 that extend on each side of the midrib in longitudinal 

 lines, which form a gentle curve, following the shape 

 of the leaf. In the smoother and more succulent 

 leaves of this division, however, these costa3 are 

 scarcely visible externally, or at least appear merely 

 as striae on the surface of the leaf : and this is the 

 case, also, as far as regards many of the smaller vas- 

 cular fasciculi, even in those leaves, which have pro- 

 minent costae. If we now make transverse and lon- 

 gitudinal sections of the Lily leaf, we perceive that 

 the costas are composed of fasciculi of spiral vessels 

 closely accompanied with corresponding fasciculi of 

 proper vessels, and imbedded in cellular substance : 

 or, that the leaf has a double system of vessels, one 

 for conducting forward the sap, and the other for re- 

 turning the proper juice into which the sap has been 

 changed by the functions of this organ. In the trans- 

 verse section, these vascular bundles appear like dots 

 upon the divided surface ; and, when magnified in 

 transmitted light, display their two-fold nature by dif- 

 ference of transparency ; the part of each fasciculus 

 composed of spiral vessels being particularly distin- 

 guished by a greater degree of opacity, owing to the 

 spiral thread which composes the coats of these ves- 

 sels being firmer and more opaque than the coats of 

 the proper vessels. The spiral vessels of the leaf as 

 well as those of the stern, are found generally empty, 

 like the arteries of animals ; while the proper or re- 

 turning vessels are always full. In the majority of 

 leaves the spiral vessels have a closer proximity to 

 the upper than to the under disk For, independent 

 of the fact, that the chief function of these organs, 



